This invention relates to a radio frequency (RF) converter circuit used in an audio tuner with RF output, a CATV converter and the like.
As shown by the block diagram of a conventional RF converter circuit in FIG. 4, a high-frequency TV signal in the VHF or UHF band received by a reception antenna is inputted through an antenna input terminal 1 and only the desired signal is selected by a tuner circuit 2 and converted into an intermediate frequency (IF) signal. After this IF signal is amplified by an IF amplifier circuit 3, it is separated by a detector circuit 4 into a video signal and an audio signal and the video signal is modulated into a one-channel or two-channel carrier wave by a RF modulator circuit 5 and taken out as a RF signal while the audio signal is converted into an audio output by an audio circuit 6 comprised of audio IF amplifier and detector circuits. Numerals 7 and 8 respectively indicate a RF automatic gain control (AGC) circuit and an IF AGC circuit adapted to receive a signal from the detector circuit 4 and output it respectively by a delay-type AGC to the tuner 2 and to the IF amplifier circuit 3. The circuit described above is structurally complicated and is expensive although many of the component circuits are integrated. Moreover, tuning is difficult because there are two AGC circuits 7 and 8 to be adjusted.